Purines. XLIX. Synthesis and Proton Nuclear Magnetic Resonance Study of 3, 7-Dialkylxanthines and 1, 3, 7-Trialkylxanthines

A general synthetic route to 3, 7-dialkylxanthines (type 9) from 3, 7-dialkyladenines (6) [hence from 3- or 7-alkyladenines (11 or 10)] has been established. The route started with ethoxycarbonylation of 1-alkyl-4-(alkylamino)-1H-imidazole-5-carboxamides (7), readily obtainable from 6 by alkaline hydrolysis, and proceeded through cyclization of the resulting carbamates (8) under alkaline conditions. Alkylation of 9 with alkyl halide in N, N-dimethylformamide in the presence of anhydrous K2CO3 extended the above synthetic route to the 1, 3, 7-trialkylxanthine level (type 14). Hydrogenolytic debenzylation of 3-benzyl-1, 7-dimethylxanthine (16), prepared by following this general synthetic route, furnished paraxanthine (26) in fair yield. Conversion of 26 into 3-(4-hydroxy-3-nitrobenzyl)-1, 7-dimethylxanthine (24), isomeric with the bryozoan purine phidolopin (2), was effected through aralkylation with 4-(methoxymethocy)-3-nitrobenzyl bromide (28) followed by O-deprotection.On the basis of proton nuclear magnetic resonance data for the 3, 7-dialkylxanthines (3 and 9b-i) and 1, 3, 7-trialkylxanthines (5 and 14-22) thus prepared, reliable criteria for distinguishing signals of N-alkyl substituents at various positions are put forward.